1. Field of the Invention
The present invention relates to a chip leveling apparatus of semiconductor wafer exposure equipment. More particularly, the present invention relates to the exposure field sensor of the chip leveling apparatus which senses the average inclination of an exposure field of the wafer and generates signals which are used by the apparatus to ensure that the exposure field is perpendicular to the optical axis of a reduction projection lens of the exposure equipment.
2. Description of the Related Art
The accurate and fine formation of electrical circuit patterns is critical in the production of highly-integrated semiconductor devices.
Such patterns are typically formed on semiconductor wafers by a stepper. The stepper has a reduction projection lens by which a specific pattern of a reticle is optically reduced and transferred onto the wafer by exposing the wafer to light passing through the reticle. In this process, the wafer surface must lie perpendicular to the optical axis of the reduction projection lens if the pattern of the reticle is to be precisely reproduced on the surface of the wafer.
The exposure process is thus gaining more scrutiny in the attempt to meet the demand for more highly-integrated semiconductor devices.
As the result of such scrutiny, it has been noted that sometimes the optical axis of the reduction projection lens is thrown out of alignment during the maintenance or repair of the equipment, or during the exposure process. Furthermore, the degree to which the surface of the wafer is level varies amongst the different portions of the surface. Therefore, certain portions of the wafer surface do not lie perpendicular to the optical axis. In other words, the exposure field is not always perpendicular to the optical axis of the reduction projection lens.
To address this problem, the stepper has a chip leveling apparatus. The apparatus performs a chip leveling operation prior to the execution of the exposure process. "Chip leveling" refers to the operation of orienting the chip surface of the wafer so that it is perpendicular to the optical axis of the reduction projection lens.
The conventional chip leveling apparatus, as illustrated in FIG. 1, comprises a leveling sensor assembly and a wafer leveling stage. The leveling sensor assembly optically examines the exposure field of the wafer and determines the average inclination of that area of the wafer.
More specifically, light emitted by a light emitting diode 10 is collimated by a collimator lens 12, and is incident on the wafer 14. The center of the beam coincides with the center of the exposure field, and the beam reflected from the wafer surface 14 passes through a collecting lens 16. The collecting lens 16 focuses the beam onto a location detector 18. The location detector 18 is typically a light sensor having four quadrants delineated by cross hairs. The circular face of the beam of reflected light is separated into four sections by the cross hairs.
The wafer leveling stage 20 is adjusted, based on the results of the detection, by the system illustrated in FIG. 2. The wafer leveling stage 20 has one fixed mount 22 and two movable mounts 24, 26. The fixed mount 22 is supported by a support column 28. Other support columns 30 are disposed below the movable mounts 24, 26, respectively. Each support column 30 is in turn supported by a conical cam 34. A stepper motor 32 is coupled with each cam 34 so as to reciprocate the cam 34 linearly in a horizontal direction.
An electrical signal, indicative of the amounts of light received in each quadrant of the light sensor, is generated by the location detector 18. This signal is issued to a focus part 36 of a controller. The focus part 36 converts the electrical signal into a signal representative of the direction in which the wafer leveling stage 20 must be inclined to level the exposure field of the wafer, and transfers the signal to a stage controlling part 38. The signal input to the stage controlling part 38 is converted into a motor-driving signal which is amplified by an amplifier 40 of the controller and issued to the stepper motors 32. Accordingly, the stepper motors 32 are driven as the situation requires. When a stepper motor 32 is driven, the cam 34 coupled thereto is moved axially. Consequently, the movable mount supported on the cam 34 via the support column 30 moves vertically.
The inclination of the wafer leveling stage 20 is dependent on the positions of the mounts 24, 26. The wafer leveling stage 20 is tilted until the exposure field of the wafer is perpendicular to the optical axis of the reduction projection lens.
FIG. 3b shows the light beam incident on a certain quadrant of the light sensor before the position of the wafer leveling stage 20 has been adjusted. When the chip leveling operation is completed, the light beam reflected from the wafer 14 will be focused on the center of the light sensor as shown in FIG. 3a.
Accordingly, an accurate detection of the spot of light by the light sensor is very important to the wafer chip leveling operation.
The beam of light generated in the conventional wafer chip leveling apparatus has a fixed diameter. However, the chip leveling operation is carried out for chips having various sizes. Therefore, the conventional chip leveling apparatus can only perform correctly for chips of certain sizes.
That is, when the diameter of the incident beam is larger than the exposure field as shown in FIG. 3c, the beam is reflected off of an area larger than the actual area set for exposure. The average inclination of the area scanned by the beam can differ from the average inclination of the exposure field. Thus, the chip leveling operation can be carried out inaccurately.
FIG. 3d shows the case of leveling a chip located at the periphery of the wafer. In this case, the beam reflected from the exposure field is truncated. As a result, the location detector 18 often creates a processing error that causes the chip leveling apparatus to malfunction.